How to efficiently drop a table with many extents
This note describes why a user process can consume large amounts of CPU
after dropping a table consisting of many extents, and a potential
workaround to stop the problem occurring. Essentially the CPU is being
used to manipulate the extents i.e. moving used extents (uet$) to free
extents (fet$). In certain circumstances it may be possible to regulate
this CPU activity.
SCOPE & APPLICATION
This article is intended to assist DBAs who may need to drop a table
consisting of many extents.
Note:61997.1 SMON - Temporary Segment Cleanup and Free Space Coalescing
Permanent object cleanup
If a permanent object (table) is made up of many extents, and the object is
to be dropped, the user process dropping the object will consume large
amounts of CPU - this is an inescapable fact. However, with some forethought
it is possible to mitigate the effects of CPU usage (and hence the knock-on
effect on other users of system resources) thus:
1. Identify, but do NOT drop the table
2. Truncate the table, specifying the REUSE STORAGE clause. This will be
quick as extents are not deallocated; the highwater mark is simply
adjusted to the segment header block.
3. Deallocate unused extents from the table, SPECIFYING THE KEEP CLAUSE.
This is the crux - you can control how many extents are to be deallocated
by specifying how much (in terms of Kb or Mb) of the table is NOT
to be deallocated.
o. Table BIGTAB is 2Gb in size and consists of 262144 8Kb extents
o. There is little CPU power available, and (from past experience) it is
known that dropping an object of this number of extents can take days
o. The system is quiet at night times (no other users or batch jobs)
In the above example the table could be dropped in 'phases' over the period
of a few nights as follows:
1. Truncate the table, specifying the REUSE STORAGE clause:
SQL> TRUNCATE TABLE BIGTAB REUSE STORAGE;
2. If it takes 3 days (72 hours) to drop the table, spread this out over
6 nights i.e. drop 1/3 Gb per night. This can be achieved in 6 (nightly)
steps as follows:
SQL> ALTER TABLE BIGTAB DEALLOCATE UNUSED KEEP 1707M; (2Gb*5/6)
SQL> ALTER TABLE BIGTAB DEALLOCATE UNUSED KEEP 1365M; (2Gb*4/6)
SQL> ALTER TABLE BIGTAB DEALLOCATE UNUSED KEEP 1024M; (2Gb*3/6)
SQL> ALTER TABLE BIGTAB DEALLOCATE UNUSED KEEP 683M; (2Gb*2/6)
SQL> ALTER TABLE BIGTAB DEALLOCATE UNUSED KEEP 341M; (2Gb*1/6)
SQL> DROP TABLE BIGTAB;
The same method can be applied if LOB segments or indexes are involved.
SQL> ALTER TABLE MODIFY LOB ()
DEALLOCATE UNUSED KEEP M;
SQL> ALTER INDEX DEALLOCATE UNUSED KEEP M;
o. If you have inadvertently tried to drop the table, this method will
not work. This is because the drop will first convert the segment to
a temporary segment, and only then start cleaning up the now temporary
segment's extents. Thus, if the drop is interrupted, the temporary
segment will now be cleaned up by SMON.
o. This method will only work for table, lob and index segment types.
o. This method will not work for segments bigger than 4gb in size due to
1190939 -- ORA-3277 WHEN ISSUING AN ALTER TABLE DEALLOCATE UNUSED > 4G (fixed in 10g and higher)
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